This invention relates to methods and systems for sensing the proximity of a rotating blade in a turbine, and to methods and systems for maintaining a substantially constant clearance of a rotating blade in a turbine.
The invention is more particularly concerned with systems for sensing the clearance between the tip of a blade in a gas-turbine engine and a surrounding member, and to systems for maintaining a constant small clearance.
For maximum efficiency in a gas-turbine engine, the clearance between the tips of the turbine blades and the surrounding part of the engine housing must be maintained as small as possible without impeding free running of the engine shaft. The large changes in temperature experienced during operation of gas-turbine engines cause the various component parts of the engine to undergo significant thermal expansion. Because of this, it is difficult to construct engines in which the clearance between the blades and the housing is small and in which the tips of the blades will not contact the housing during prolonged operation.
It has been proposed to measure the clearance between the tips of the blades and the surrounding turbine ring, or shroud, with some form of sensor system and to alter the relative position of the blades with respect to the surrounding member in accordance with the measure of clearance, such as to maintain a constant small clearance.
More particularly, it has been proposed, in U.K. Pat. No. 1 545 656, to use a microwave proximity sensor to measure the turbine blade clearance. The system described in U.K. Pat. No. 1 545 656 uses a magic tee waveguide, one port of which is coupled to a microwave power source. The opposing second and third ports in the colinear arms are coupled respectively to a resonant waveguide iris and to an adjustable short-circuit device. The fourth port is coupled to a microwave detector. Propagating incident waves generated by the microwave source are divided equally between the two colinear arms, none of the incident waves passing to the fourth port. The resonant iris is designed to produce propagating reflected waves on the inner side of the iris (within the waveguide) and evanescent or non-propagating electromagnetic fields on the other side of iris, to the outside of the waveguide. The waveguide is mounted such that the iris is flush with the inner surface of that part of the engine housing surrounding the turbine blades. As the tips of the blades rotate past the iris, they perturb the evanescent electromagnetic fields and detune the iris by altering the equivalent shunt capacitance or inductance. Reflected waves from the iris now interfere with those from the short-circuited port and thereby propagate through the fourth port where they are detected. The output of the detector is dependent upon the reflection coefficient of the iris which is in turn dependent upon the separation of the iris from the tip of the turbine blades. By measuring the output of the detector it is thereby possible to obtain a measure of the turbine blade clearance.
In this microwave proximity sensing system, the short circuit at the second port is initially adjusted to achieve a balance and a nil output at the detector. If accuracy is to be achieved this balanced must not be affected during operation by factors other than passage of the turbine blades. To maintain the balance, however, the attenuation of the two colinear arms and their electrical lengths must be maintained equal and, since the attenuation and length of both arms will vary with their respective temperatures, it would be necessary with a gas-turbine engine to ensure that the short-circuited arm is maintained at the same temperature as the arm terminated by the iris. It can be seen that this will be difficult to achieve because the arm terminated by the iris will be subjected to the high temperatures and temperature variations in the region of the engine's turbine blades. Even if the waveguide could be held at the same temperature throughout, this would still result in variations in sensitivity of the system arising from variations in the overall temperature of the waveguide.